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One out of three American adults is obese, and childhood obesity has more than doubled in the past 30 years. It’s an epidemic with very serious health risks, from heart disease to diabetes to cancer. It’s also taking an incredible financial toll: Studies have put the national healthcare cost of obesity at over $190 billion a year.

Behind these staggering figures are measurement tools that the medical community uses to determine who is at risk. One of the most common is body mass index (BMI). The problem is that BMI alone doesn’t tell the whole story.

BMI is a mathematical equation — weight (in pounds) divided by height (in inches) squared, multiplied by 703 — to calculate an individual’s body fat. An adult with a BMI of 30 or higher, for instance, is considered obese. But because BMI is based on body weight rather than body composition, it doesn’t take into account how much of that weight is from muscle, bone or water as opposed to fat.

“BMI is a useful number in general terms, but it doesn’t tell us where the fat is distributed or distinguish between fat and muscle,” said Subhashini Ayloo, MD, a bariatric surgeon at University of Illinois Hospital. That means an athlete who has more muscle mass and strong bones could have a high BMI that would categorize them as overweight or even obese. On the other hand, older people who have lost muscle may have a deceptively lower BMI.

While the Centers for Disease Control and Prevention considers BMI “a reasonable indicator of body fat,” it doesn’t recommend it as a diagnostic tool.

“It’s not perfect but it’s definitely a useful tool,” said Ellen Demerath, an associate professor of epidemiology and community health at the University of Minnesota. “It should be the first step in measuring an individual’s health assessment, but other tests can also be useful.”

One simple alternative to BMI is measuring waist circumference which, as Demerath points out, “can be an indicator of risk for many obesity-related diseases.”

Men with a waist circumference of more than 40 inches and women with a circumference of more than 35 inches are at a higher risk for developing obesity-related conditions like type 2 diabetes, high blood pressure and high cholesterol. In a new study out this month, so-called “apple-shaped” people who carry more fat around their bellies are also at a higher risk for kidney disease.

“If someone has a pear shape and their weight is in the hips and thighs, which is classically a female body shape, they don’t have the same risk factors,” said Eric Westman, MD, an internist at Duke Health Center in Durham, N.C.

Another method of measuring fat that’s becoming more popular and accessible is bioelectrical impedance — a device resembling an ordinary scale that measures body composition by sending a low electrical current from footpad electrodes. “It’s a more accurate way of measuring the actual percentage of fat,” Dr. Westman said. “You see them a lot now in health clubs, and they’re available for home use.”

BMI is still a valid and inexpensive screening tool. But it’s just one piece of the puzzle. The National Heart, Lung, and Blood Institute recommends that any weight-related health assessments should be based on a combination of BMI, waist circumference and other individual risk factors such as high blood pressure, cholesterol or physical inactivity.

“The guidelines for determining obesity are not hard and fast rules, they’re a consensus that most doctors agree on,” Dr. Ayloo said. “Right now one of the criteria used is BMI because it’s easily accessible, but it’s not perfect on its own.”